1. Field of the Invention
The present invention relates to a backlight for a display device, and more particularly, to a backlight for a liquid crystal display (LCD) device.
2. Discussion of the Related Art
In recent days, there has been an increase in demand for flat panel display devices having thin, light weight, and small profiles for applications in portable electronic devices, such as mobile phones, personal digital assistants (PDAs), and notebook computers. Examples of these flat panel display devices include liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays (FEDs), vacuum fluorescent displays (VFDs), and the like. Among the flat panel display devices, the LCD device is the most popular due to mass productions of the device, relatively simple method for driving these devices, and high picture quality.
The LCD device displays a desired image on a screen by controlling an amount of light passing through a liquid crystal layer due to refraction anisotropy of the liquid crystal molecules. Accordingly, a backlight, which act as a light source that emits light through the liquid crystal layer, is installed in the LCD device for an image display. Backlights are generally classified into two categories: (1) an edge-type and (2) a direct-type. An edge-type backlight has a light source installed along a side surface of a liquid crystal panel to provide light to the liquid crystal layer. A direct-type backlight has a light source installed at a lower portion of the liquid crystal panel to directly provide light to the liquid crystal layer.
The edge-type backlight is installed at a side surface of a liquid crystal panel and provides light to the liquid crystal layer through a light guide plate and a reflector. Since the edge-type backlights are slim, they are mainly used in systems requiring thin display devices, such as notebook computers and the like. However, since the edge type backlights are positioned at a side surface of a liquid crystal panel, it is difficult to provide uniform light to a large area. Further, because the light is provided to the liquid crystal layer through a light guide plate, edge-type backlights have difficulty providing lighting with high brightness. Accordingly, the edge-type backlight is not suitable for large liquid crystal panels, such as large LCD televisions being recently marketed.
On the other hand, the direct-type backlights emit light directly to the LCD panel from a lamp of lamps, thereby being adapted to be applied to large liquid crystal panels. Further, since the direct-type backlight provides lighting with high brightness, it is mainly used with liquid crystal panels for LCD televisions.
FIG. 1 shows an LCD device to which a direct-type backlight is applied in accordance with the related art. As shown in FIG. 1, an LCD device 1 includes a liquid crystal display panel 3, and a backlight 10 installed at a rear surface of the liquid crystal panel 3. The liquid crystal panel 3 for displaying an image includes a transparent lower substrate 3a, such as a glass, an upper substrate 3b, and a liquid crystal layer (not shown) formed therebetween. Although not shown, the lower substrate 3a is a thin film transistor substrate having formed thereon driving devices such as thin film transistors (TFTs) and pixel electrodes. The upper substrate 3b is a color filter substrate having a color filter layer formed thereon. Also, a driving circuit unit 5 for respectively applying signals to the thin film transistors and the pixel electrodes is formed at a side of the lower substrate 3a. The backlight 10 includes a plurality of lamps 11 for emitting light and supplying the light to the liquid crystal panel 3, a reflector 17 for reflecting light emitted from the lamps 11 and enhancing an optical efficiency, and an optical sheet 15 for diffusing light emitted from the lamps 11 and directing the light to be incident on the liquid crystal panel 3.
FIG. 2 is a block diagram schematically showing a circuit structure of the backlight 10 according to a related art. As shown, the lamps 11 are provided with transformers T1 at both ends thereof. The transformer T1 is connected to an A/D converter 22, such as a bridge circuit. An alternating current inputted from an input voltage (Vin) terminal is converted into a direct current by the A/D converter 22 and is applied to the transformer T1. Then, a controlled voltage of the current applied to the transformer T1 is applied to both ends of each lamp 11.
The A/D converter 22 is connected to a controlling unit 20 and converts an alternating current into a direct current according to a control signal supplied from the controlling unit 20. A current outputted from the lamps 11 is inputted to the controlling unit 20 as a feedback signal. The controlling unit 20 compares the inputted current with a preset current. The controlling unit 20 compensates the inputted current based on the comparison and then supplies the compensated current to the lamps 11.
However, as shown in FIG. 2, the plural controlling units 20 and the A/D converters 22 are respectively connected to two lamps 11. That is, in the related art backlight, the current supplied to the lamps 11 are fed back to each individual controlling unit 20 to individually drive the pair of lamps 11. Accordingly, the current individually supplied to each pair of lamps 11 may be different from each other, thereby causing difference in brightness of the emitted light. The difference in brightness reduces picture quality when driving an LCD device.